Preparation Of TiO₂ Nanomaterials With Exposed {001} Crystal Planes And Research On Its Gas-sensitive Properties

The exposed crystal facet of nanomaterials is one of crucial factors affecting their physical and chemical properties.However,the highly reactive facets are usually disappeared during crystal growth because of the minimization of the surface free energy.Thus,the controllable syntheses of inorganic single crystals with highly reactive facets have become difficult point and research hotspot.TiO₂ has been intensively used in energy and environmental fields due to its unique structure and physicochemical properties.Particularly,TiO₂ with highly reactive {001} facets has attracted worldwide research interest.Nevertheless,most of the reported works have been concentrated on their controllable preparation and application in photocatalysis,and the application of TiO₂ nanocrystals with exposed {001} facets in gas sensing is still in its babyhood.Therefore,it is highly essential to explore the gas sensing properties based on TiO₂ nanocrystals with exposed {001} facets.In this paper,TiO₂ nanocrystals with different percentages of exposed {001}facets were fabricated by a hydrothermal approach by using HF as morphologic control agent.The gas sensing properties of TiO₂ nanocrystals with exposed {001} facets were explored.Abnormal p-type sensing behavior at low temperatures were revealed.for TiO₂ nanocrystals.Importantly,both the p-type and n-type sensing responses were facet-dependent.The main research results were as follows:Firstly,TiO₂ nanocrystals with different percentages of exposed {001 }facets were controllable prepared by adjusting the volumes of HF solution.When the amounts of HF were 0 mL,1.5 mL,2 mL and 3 mL,the percentages of exposed {001} facets were estimated to be 24.8%,36.4%,49.7%and 76.5%,respectively.It should be noted that when the amount of HF increased further,the percentage of {001} facets reduced.Therefore,TiO₂ sample prepared with 3 mL HF had the highest percentage of {001}crystal facets.Secondly,abnormal p-type sensing behavior was exhibited over a wide temperature range from room temperature to 120 ℃ based on TiO₂ nanosheets prepared with 3 mL HF.Such an abnormal p-type sensing behavior,unprecedented for n-type TiO₂ nanomaterials,can be ascribed to the proton transfer between alcohol molecules and adsorbed water molecules on the surface of TiO₂ nanosheets.Then,the gas-sensing performances towards ethanol of the sensors constructed by TiO₂ nanocrystals with different percentages of exposed {001} facets were investigated detailedly.A regular n-type sensing behavior to ethanol above 250 ℃ and abnormal p-type response at low temperatures（room temperature-120 ℃）were exhibited for TiO₂ nanocrystals.Importantly,crystal facet-dependent p-type and n-type sensing responses were revealed,illustrating an evident increase of response with increasing percentage of exposed {001} facets.Finally,Some new ideas were provided for the crystal facet-dependent p-type and n-type gas sensing mechanism of TiO₂ nanomaterials.The facet-dependent p-type sensing mechanism was attributed to the increased water molecules on TiO₂ surface with increasing {001} percentage,while the facet-dependent n-type sensing mechanism was ascribed to the increased adsor:ption capabilities of ethanol molecules and surface oxygen species with increasing {001} percentage.In addition,the change of oxygen-absorbed ion from O-to O₂-with increasing {001} percentage also contributed to the facet-dependent n-type sensing properties.The present work here offered some new insights into the facet-dependent gas sensing mechanism,which was helpful to design highly active sensing materials by exposing specific crystal facets.